let letters = [..."hello, world"];
let string = "";
for (let letter of letters) {
  string += letter;
}
console.log(string); // hello, world
let a = ["a", , "b"];
for (let v of a) {
  console.log(v); // a undefined b
}
// index 为索引
for (let [index, v] of a.entries()) {
  // 0 a \n 1 undefined \n 2 b
  console.log(index, v);
}
// forEach 能够感知稀疏矩阵，不会对没有的元素数组调用函数
a.forEach((v, i) => console.log(i, v)); // 0 a \n 2 b
